This invention concerns the making of welded joints between thin sections of light metals such as aluminum and, more particularly, to the welding of attachments such as mounting brackets of thin section to external surfaces of hollow thin walled elongated members, such as lengths of thin walled aluminum extrusions.
Modern stress analysis techniques, materials, and advances in foundry and welding processes make feasible the production of structures which are very efficient in terms of strength to weight ratio. Such structures are commercially attractive especially for aircraft and automobiles where fuel economy (coupled with generous payload) remains an important goal.
It is characteristic of these efficient structures that frame members may have very thin walls and mating or attaching members including castings may be of very thin section. For example, it is sometimes desirable to join by welding a casting with section thickness at the welded joint of two to three millimeters to an aluminum extrusion with a wall thickness of less than two millimeters.
Successful execution of welded joints in structures of this nature depends in large part on dimensional and form control so as to ensure minimum variation in fit-up at the proposed welded joints. Cost considerations require the use in assembly of hollow sections such as aluminum extrusions with quite wide manufacturing variations in general shape of cross section and in particulars such as straightness of walls in the cross section. If there are significant variations in fit-up at the proposed joint, highly skilled welders, able to compensate for these variations, must be used to achieve production welds of acceptable quality and tolerable levels of scrap and repair costs. Where there is inconsistent contact between the faying surfaces at the weld joint, catastrophic burn through and/or melt through is common. And even with gas metal arc (GMA) welding such joints are unforgiving to changes in welding conditions (for example, welding current, wire feed rate, speed of travel and heat accumulation in the extrusion). It is difficult to maintain control over weld penetration and bead shape and welding rate tends to be slow. On the other hand, predictable consistency in contact between the faying surfaces of a proposed joint is less demanding of welding technique and lower grade operators may be used; and automatic or robotic execution of welding such joints becomes more economically feasible.